We used a solar charger from Tractor Supply at first for my two mares, 2 acre one I think. They respected it for a while, but as soon as winter coats came in they felt nothing and went right through it. Once they learned that, forget it. Luckily my whole property is fenced with wire field fence, so they weren't going anywhere, but it made for problems anyway. I found the solar charger simply wasn't reliable all around, and as time went on, it seemed to loose its kick. We did only have one ground rod however. We switched to an AC charger (my husband is an electrician, so setting this up was no issue, and we have power in the barn) and it is fantastic. The girls do not so much as look at the fence in an attempt to go through it, and they are in full on winter woolies now. We did sink another ground rod, so I do feel that this helped, and as many as three, as recommended by manufacturers would be better still, but a challenge in our rocky soil. In terms of electric bill, not noticeable for us, maybe a few dollars a month. Peace of mind outweighs this for me. As you probably know, and I learned quickly, minis are little houdinis, so safe fence is paramount. Good luck, having to move your horses must be stressful!!
Katie